Electrode for an electric double layer capacitor and process for producing it

ABSTRACT

A process for producing an electrode for an electric double layer capacitor, which comprises extruding a mixture comprising a carbonaceous material, a polytetrafluoroethylene and a processing aid by screw extrusion, and rolling the obtained extruded product by rolling rolls to form it into a sheet shape.

[0001] The present invention relates to a process for producing anelectrode for an electric double layer capacitor.

[0002] The demand for an electric double layer capacitor utilizing anelectric double layer formed at the interface between a polarizableelectrode and an electrolyte, particularly for a coin shaped one, israpidly increasing recently, as a memory backup power source. On theother hand, it is desired to develop an electric double layer capacitorhaving a large capacitance per volume, small internal resistance, a highenergy density and a high output density, also for a use which requiresa large capacity such as a driving source for electric cars. Further,with regard to an electric double layer capacitor for memory backup, itis desired to reduce the internal resistance.

[0003] The electrode for an electric double layer capacitor is prepared,for example, by kneading an activated carbon powder with a solvent foran electrolytic solution such as sulfuric acid, and forming the mixtureinto a slurry, followed by press forming (U.S. Pat. No. 3,288,641).However, the electrode obtained by the process has a stiff porousstructure and thus is likely to be cracked or broken, and it can not beused for a long period of time.

[0004] On the other hand, a carbon-based electrode has been proposedwhich is made of a viscous material having a binder made of apolytetrafluoroethylene (hereinafter referred to as PTFE) added to amixture comprising an activated carbon powder and an electrolyticsolution, as the case requires (JP-B-53-7025, JP-B-55-41015). Theelectrode has crack resistance and break resistance. However, it isinadequate in the shape keeping property, and a cell having a specialstructure is required to supplement the strength to use the electrode.

[0005] In order to obtain an electrode having crack resistance and breakresistance, and an excellent shape keeping property, a process has beenproposed, which comprises preliminarily molding a kneaded materialcomprising a carbonaceous material, a binder such as PTFE and a liquidlubricant, followed by stretching or rolling to obtain a formedelectrode of sheet shape (JP-A-63-107011, JP-A-2-235320).

[0006] However, by this process, PTFE is randomly formed into fibers bykneading, and a part of PTFE is formed into fibers and the rest is not,and consequently the hardness of the two parts will be different.Therefore, when forming the sheet electrode into a thin film sheethaving, for example, a thickness of at most 0.2 mm, the surface tends tobe irregular, and holes are likely to be formed. Therefore, thecapacitance per volume (hereinafter referred to as capacitance density)of the electric double layer capacitor can not be made large, and theinternal resistance is large.

[0007] A process has also been proposed, which comprises mixing anactivated carbon powder and PTFE to obtain a paste, coating the paste ona current collector, followed by drying, heating at a temperature higherthan the melting point of PTFE, and press-forming the electrode to makeit thin to increase the density (JP-A-9-36005). However, with thisprocess, the production steps are complicated, and it is difficult tocontinuously conduct the process, and a part of PTFE melts so that theinternal resistance will be high.

[0008] Further, by the above processes, it is difficult to prepare anelongate electrode sheet, and the electrode can not be obtained bycontinuous operation, whereby production efficiency is poor.

[0009] On the other hand, as a method for extruding PTFE, a pasteextrusion method has been known, which comprises using a polymer (finepowder) obtained by coagulating and drying an aqueous dispersion of PTFEmade by emulsion polymerization of a tetrafluoroethylene, adding aprocessing aid such as naphtha or white lamp oil thereto, preliminarilymolding the mixture to form it into a sleeve shape and filling it to acylinder mold, followed by pressurizing by a ram to extrude it through anozzle suitable for a shape of a rod or a sheet, and vaporizing theprocessing aid to obtain a formed product (JP-B-61-54578).

[0010] This method is generally applied to molding of PTFE alone or PTFEcontaining several wt % of a filler, and is not applied to moldingwherein a filler is the main component and PTFE is used merely as amechanical supplement. Namely, since the filler such as graphite, glassfiber or carbon fiber is not likely to undergo plastic deformation, in acase where it is mixed with PTFE and molded, the extrusion pressuretends to be high, and PTFE is highly deformed. Therefore, there is aproblem that the obtained extruded product is fragile and has lowstrength. Further, in the paste extrusion method, the amount extruded ata time depends on the shape of sleeve, and thus it is difficult tocontinuously obtain an elongate extruded product.

[0011] The present invention has been made to overcome theabove-mentioned problems of the prior art, and it is an object of thepresent invention to provide an electrode having a shape of thin filmsheet and having high strength, and a process for continuously producingthe electrode, thereby to provide an electrode for an electric doublelayer capacitor having a high capacitance density and small internalresistance, particularly an electrode for an electric double layercapacitor suitable for use wherein a large current is required with ahigh capacitance.

[0012] The present invention provides a process for producing anelectrode for an electric double layer capacitor, which comprisesextruding a mixture comprising a carbonaceous material, apolytetrafluoroethylene and a processing aid by screw extrusion, androlling the obtained extruded product by rolling rolls to form it into asheet shape, and an electrode obtainable by the process.

[0013] As the carbonaceous material, a powder of e.g. activated carbon,polyacene or carbon black, which has a specific surface area of from 200to 3500 m²/g, is preferred. Further, a fiber or a powder of e.g. carbonfiber, carbon whisker or graphite can also be preferably used, so longas its specific surface area is from 200 to 3500 m²/g. As the activatedcarbon, a phenol type, a rayon type, an acryl type, a pitch type or acoconut shell type may be used. The particle size is preferably from 0.1to 100 μm, particularly preferably from 1 to 20 μm, whereby it is easyto form the electrode into a thin film sheet, and the capacitancedensity can be made high.

[0014] It is also preferred to use carbon black in admixture withanother carbonaceous material, as a conductive material. The particlesize of the carbon black is preferably from 0.001 to 1 μm, particularlypreferably from 0.01 to 0.5 μm, and the specific surface area of thecarbon black is preferably from 200 to 1500 m²/g, particularlypreferably from 500 to 1300 m²/g. An electrode comprising the carbonblack as a conductive material, activated carbon having a specificsurface area of from 200 to 3500 m²/g and a particle size of from 0.1 to100 μm, and PTFE, is preferred since the internal resistance can be keptlow, and the capacitance can be kept high.

[0015] In the present invention, PTFE includes not only a homopolymer oftetrafluoroethylene but a copolymer obtained by adding at most 0.5 mol %of another monomer to tetrafluoroethylene, followed by copolymerization.When another monomer is at most 0.5 mol %, the melt fluidity is notgiven to PTFE, and it is possible to form the copolymer into fibers andto prepare an electrode sheet having high strength and low resistance,just like the homopolymer of tetrafluoroethylene. As another monomer,hexafluoropropylene, chlorotrifluoroethylene, perfluoro(alkyl vinylether), trifluoroethylene or (perfluoroalkyl)ethylene may, for example,be mentioned.

[0016] PTFE is a liquid or a gel when it is of low molecular weight, andin such a state, it can hardly be formed into fibers. Therefore, it ispreferred that PTFE contains at least 50 wt % of a polymer having amolecular weight of at least 1×10⁶ as calculated from the standardspecific gravity. Further, PTFE obtained by emulsion polymerization ispreferred since it is easily formed into fibers.

[0017] The processing aid of the present invention is added so that PTFEis suitably formed into fibers and undergoes plastic deformation. It isnot particularly limited so long as it is a liquid capable of wettingPTFE easily, and being removed easily from the formed electrode.Specifically, a hydrocarbon such as ethanol, methanol, 2-propanol, lampoil, solvent naphtha or white naphtha, ethylene glycol, propyleneglycol, dipropylene glycol or glycerol may, for example, be mentioned.Further, a dispersion of a fluoro resin such as an aqueous dispersion ofPTFE may be used as the processing aid, and it may be used alone or incombination with another processing aid. Particularly, propylene glycolor dipropylene glycol is preferably used.

[0018] In the process of the present invention, PTFE, the carbonaceousmaterial and the processing aid are mixed so that PTFE is contained inthe electrode in an amount of preferably from 1 to 50 wt %, morepreferably from 5 to 30 wt %, to the carbonaceous material. Sine PTFE iscontained in the electrode as a binder to keep the shape of theelectrode sheet, if PTFE is less than 1 wt %, the strength tends to below, and if PTFE is more than 50 wt %, the internal resistance of theelectrode tends to increase.

[0019] In the process of the present invention, the processing aid maybe added after mixing the carbonaceous material and PTFE or when mixingthem. The mixture comprising the carbonaceous material, PTFE and theprocessing aid may be a granulated product, but it does not influencethe screw extrusion. It is preferred to add from 20 to 200 wt %,particularly from 40 to 100 wt %, of the processing aid to thecarbonaceous material. If the processing aid is less than 20 wt %, thepressure for extrusion tends to be high, thereby the extrusion becomesdifficult. If the processing aid is more than 200 wt %, the pressure forextrusion does not increase, PTFE tends to be not adequately formed intofibers, and the processing aid is likely to exude from gaps of the moldor the cylinder, or flow backward and flow out from the opening of thehopper during extrusion.

[0020] In the process of the present invention, screw extrusion by usinga screw extruder is conducted. The screw extruder has a screw havingspiral grooves, rotatable in the cylinder of the extruder, and kneadsthe mixture comprising a carbonaceous material, PTFE and a processingaid while moving the mixture in the cylinder by rotation of the screw,and then extrudes it. In the present invention, a screw extruder forresin, for rubber or for building material can be used. Further, amonoaxial extruder or a multiaxial extruder may be used. As the shape ofthe screw, various shapes such as a full flight screw, a variable pitchscrew and a screw equipped with a mixing pitch may be employed. A fullflight screw with which the adjustment of extrusion pressure is easy, isparticularly preferred.

[0021] The extrusion temperature for the screw extrusion is preferablyfrom 5 to 300° C., particularly preferably from 30 to 150° C. If it isless than 5° C., the extruded product tends to be fragile, whereby it isdifficult to keep the shape, and if it is higher than 300° C., theprocessing aid will significantly evaporate, whereby extrusion becomesdifficult.

[0022] The extrusion pressure for the screw extrusion is preferably from2 to 300 kg/cm², more preferably from 5 to 100 kg/cm², particularlypreferably from 5 to 60 kg/cm². If it is less than 2 kg/cm², theextruded product tends to be too soft, whereby it is difficult to keepthe shape, and if it is higher than 300 kg/cm², the extrusion tends tobe difficult, and the obtained extruded product tends to be fragile.

[0023] The screw compression ratio in the screw extrusion is preferablyfrom 1.0 to 4.0, particularly preferably from 1.0 to 2.0. If it is lessthan 1.0, PTFE is not adequately formed into a fibers, the strength ofthe extruded product is not adequate, whereby it is difficult to keepthe shape of the electrode. If it is more than 4.0, the extrusionpressure tends to be high, whereby the extrusion becomes difficult.Here, the screw compression ratio is a value obtained by dividing thecross-sectional area between the inside of the cylinder and the groovediameter of the screw at the base of the screw, by the cross-sectionalarea between the inside of the cylinder and the groove diameter of thescrew at the top of the screw.

[0024] In the present invention, the screw-extruded product is thenrolled by rolling rolls (hereinafter referred to as rolling), to form itinto a sheet shape. The thickness of the screw-extruded product becomesusually from about {fraction (1/10)} to about {fraction (1/200)} byrolling. PTFE in the extruded product by the screw extrusion is formedinto fibers in the direction perpendicular to the extrusion direction.Therefore, the rolling treatment is conducted preferably in the samedirection as the extrusion direction of the screw extrusion, since theforming into fibers in the extrusion direction is accelerated, PTFE isformed into fibers in both length and breadth directions to have anetwork structure, and the carbonaceous material is kept by the networkstructure, whereby the strength can be made high.

[0025] The rolling temperature for rolling is preferably from 20 to 350°C., particularly preferably from 60 to 150° C. If the rollingtemperature is lower than 20° C., PTFE is less likely to be formed intofibers adequately, whereby the sheet tends to be fragile. If the rollingtemperature is higher than 350° C., the processing aid willsignificantly evaporate, whereby cracking or separation is likely toresult on the surface of the sheet. Here, the rolling temperature meansthe temperature of rolling rolls when the rolling rolls contact thesheet.

[0026] The sheet formed by rolling is then dried to remove theprocessing aid. The temperature for drying is preferably higher than theboiling point of the processing aid and lower than the melting point ofPTFE. Further, it is possible to stretch the sheet-shaped product whichis dried or semi-dried wherein the processing aid is partially removed.Further, it is possible to conduct rolling after stretching.

[0027] In the case of stretching, the stretching ratio is preferablyfrom 1.01 to 5.0 times, and the stretching is conducted monoaxially ormultiaxially. Further, the stretching may be conducted before the dryingstep. By stretching, the forming of PTFE into fibers is accelerated, anda thin film sheet having high strength and low resistance can beobtained. The temperature during stretching is preferably from 30 to350° C., particularly preferably from 200 to 320° C., whereby theforming of PTFE into fibers can be more accelerated.

[0028] The electrode sheet obtained by the process of the presentinvention can be used as an electrode as it is initially formed.However, it may be used after baking, as the case requires. The bakingmay be complete baking at a temperature higher than the melting point ofPTFE or incomplete baking at a temperature lower than the melting pointof PTFE.

[0029] By the process of the present invention, an electrode sheet canbe obtained, wherein PTFE is formed into fibers in both length andbreadth directions to have a network structure, and the carbonaceousmaterial is kept by the network structure of PTFE. The electrode hashigh strength and excellent shape keeping property, and it is easy toform the electrode into a thin film. Further, the forming of PTFE intofibers in both length and breadth directions is accelerated.Accordingly, even when the amount of PTFE is small, the electrode sheetformed into a thin film, is excellent in crack resistance, breakresistance and shape keeping property.

[0030] Further, in the present invention, it is possible to conductcontinuous extrusion by screw extrusion, whereby a continuous electrodewhich is long in extrusion direction, can be obtained. By rollingtreatment of the extruded product, an elongate electrode can beobtained.

[0031] Further, since PTFE is formed into fibers and has athree-dimensional network structure, the increase of the resistance ofthe electrode by blending PTFE which is non electroconductive, is small.Further, forming of the three-dimensional network structure isaccelerated by forming of PTFE into fibers, and thus the resistance ofthe electrode will further decrease. Further, in the case where thecarbon black is added as a conductive material, a high pressure isexerted on carbon black in both steps of screw extrusion and rolling,whereby the electrode will have low resistance by electrical connectioneven with a small amount of carbon black.

[0032] With regard to the strength of the electrode sheet of the presentinvention, the tensile strength is preferably at least 1.5 kg/cm²,particularly preferably at least 2.0 kg/cm² in at least one direction.In the process of the present invention, in a case where the electrodeis produced so that the direction of screw extruding and the directionof rolling are the same, the tensile strength in one direction asdescribed above corresponds to the tensile strength in the direction ofrolling.

[0033] In the present specification, the tensile strength of anelectrode is a value obtained by dividing the maximum load, when theelectrode sheet is dried for an hour at a temperature of 250° C.,punched to a shape of a dumbbell specimen of No. 1 as stipulated in JISK6301, and subjected to a tensile test at a pulling rate of 20 mm/min atan atmosphere temperature of 25±2° C., by the cross-sectional area (thethickness of the electrode sheet×the width of the parallel parts).

[0034] The electrode obtained in the present invention is used for anelectric double layer capacitor as an electrode assembly which isintegrated with a current collector by e.g. bonding to the currentcollector. As the current collector, a metal foil, particularly analuminum foil is preferred. An aluminum foil may be withdrawn from acoil made by winding an elongate foil in a shape of a roll. Theelectrode which is formed by the final rolling, is integrated with analuminum foil, as soon as it is formed, from the formed portion, toconduct forming of the electrode and integration continuously on oneline. When an electrode is formed by such a continuous line, productionefficiency is high.

[0035] The integration of the electrode and the current collector ispreferably conducted by means of an adhesive between them, since theyare thereby strongly integrated. It is preferred to incorporate e.g. ahighly electroconductive powder to the adhesive, since the contactresistance between the electrode and the current collector decreases. Byrolling after bonding of the electrode and the current collector, theyare more strongly integrated. It is possible to conduct bonding afterthe processing aid is removed by drying after forming. However, it ispreferred to bond the electrode in a semi-dried condition with thecurrent collector, conduct rolling, and then remove the processing aid,as it is thereby easy to integrate the electrode and the currentcollector by rolling.

[0036] Now, the present invention will be described in further detailwith reference to Examples. However, it should be understood that thepresent invention is by no means restricted to such specific Examples.

EXAMPLE 1

[0037] To a mixture comprising 80 wt % of a high purity activated carbonpowder having a specific surface area of 1500 m²/g and an averageparticle size of 10 μm, 10 wt % of carbon black, and 10 wt % of PTFEpowder, propylene glycol was added in an amount of 66 wt % based on thetotal amount of the activated carbon powder and carbon black, followedby mixing. The mixture was extruded by screw extrusion, by using amonoaxial extruder comprising a cylinder having an inner diameter of 40mm, a full flight screw having a screw compression ratio of 1.6, and anozzle designed so that the shape of the extruded product would becylindrical (outer diameter: 102 mm, inner diameter: 86 mm), at acylinder temperature of 80° C. and a nozzle temperature of 90° C. Theextrusion pressure was 20 kg/cm².

[0038] The obtained cylindrical extruded product was cut in thelongitudinal direction of the cylinder (extrusion direction), thenopened and made flat. Then, it was rolled in the same direction asextrusion direction at a rolling temperature of 80° C., followed bydrying at a temperature of 250° C. for 60 minutes to remove propyleneglycol, and a sheet having a thickness of 120 μm was formed.

[0039] The sheet was dried at a temperature of 250° C. for one hour,punched into a shape of a dumbbell specimen of No. 1 form as stipulatedin JIS K6301, and subjected to a tensile test at a pulling rate of 20mm/min at an atmospheric temperature of 25° C. to measure the maximumload. The measurement was repeated three times, and the average valuewas taken as the maximum load applied to the sheet. In order to measurethe tensile strength of the sheet in the direction of rolling, thedumbbell specimen was punched so that the longitudinal direction was thedirection of rolling. The tensile strength of the sheet calculated fromthe value of the maximum load was 8.7 kg/cm².

[0040] To one side of a pure aluminum foil having a rectangular shapewith a width of 4 cm and a length of 6 cm, and a thickness of 50 μm, andhaving a lead terminal, an electrode sheet obtained by punching theabove sheet into an area of 4 cm×6 cm, was bonded by means of aconductive adhesive, followed by heat curing of the adhesive to obtainan electrode assembly. Two sheets of such electrode assemblies wereprepared, and arranged so that the electrode sides of the electrodeassemblies were faced each other. A glass fiber separator having athickness of 40 μm was interposed between the electrode assemblies,which were then sandwiched between two glass plates having a thicknessof 2 mm, a width of 5 cm and a length of 7 cm, to obtain an element. Thetotal thickness of the two electrode assemblies and the separator was0.39 mm.

[0041] As an electrolytic solution, a solution having 1.5 mol/l oftriethylmonomethylammonium tetrafluoroborate dissolved in propylenecarbonate, was used. The element was subjected to vacuum heating for 3hours at a temperature of 200° C., to remove impurities such as waterfrom the element, and then it was impregnated with the electrolyticsolution under vacuum and then accommodated in a polypropylenerectangular bottomed cylindrical container, to obtain an electric doublelayer capacitor. The direct current resistance and the capacitance weremeasured at a current density of 20 mA/cm², and the capacitance pervolume (capacitance density) and volume resistance were calculated. Theresults are shown in Table 1.

EXAMPLE 2

[0042] A sheet was prepared in the same manner as in Example 1, exceptthat the thickness was made to be 70 μm by rolling by rolling rolls. Thetensile strength of the sheet in the direction of rolling was measuredin the same manner as in Example 1 and found to be 1.6 kg/cm². Anelectric double layer capacitor was prepared in the same manner as inExample 1, except that an electrode sheet obtained from the sheet wasused, and the same evaluation as in Example 1 was conducted. The resultsare shown in Table 1.

EXAMPLE 3

[0043] A sheet was prepared in the same manner as in Example 1, exceptthat the thickness was made to be 150 μm by rolling by rolling rolls,the processing aid was dried, and stretching was conducted under astretching ratio of 1.5 times at a temperature of 300° C. to make thethickness to be 110 μm. The tensile strength of the sheet in thedirection of rolling by rolling rolls was measured in the same manner asin Example 1 and found to be 9.0 kg/cm². An electric double layercapacitor was prepared in the same manner as in Example 1, except thatan electrode sheet obtained from the sheet was used, and the sameevaluation as in Example 1 was conducted. The results are shown in Table1.

EXAMPLE 4

[0044] A sheet having a thickness of 90 μm was obtained in the samemanner as in Example 1, except that dipropylene glycol was added in anamount of 61 wt % based on the carbonaceous material instead ofpropylene glycol, followed by mixing. When preparing the sheet, theextrusion pressure in screw extrusion was 34 kg/cm². The tensilestrength of the sheet in the direction of rolling by rolling rolls was11.4 kg/cm², and the handling was easy as compared with the electrodesheet of Example 1. An electric double layer capacitor was prepared inthe same manner as in Example 1, except that an electrode sheet obtainedfrom the sheet was used, and the same evaluation as in Example 1 wasconducted. The results are shown in Table 1.

EXAMPLE 5 Comparative Example

[0045] A sheet was prepared in the same manner as in Example 1, exceptthat forming was carried out by roll kneading without conducting screwextrusion, then rolling by rolling rolls was repeated 5 times to makethe thickness to be 200 μm. The tensile strength of the sheet in thedirection of rolling was measured in the same manner as in Example 1 andfound to be 1.0 kg/cm². An electric double layer capacitor was preparedin the same manner as in Example 1, except that an electrode sheetobtained from the sheet was used, and the same evaluation as in Example1 was conducted. The results are shown in Table 1.

EXAMPLE 6 Comparative Example

[0046] A sheet having a thickness of 130 μm was obtained in the samemanner as in Example 1, except that forming was carried out by rollkneading without conducting screw extrusion, and then rolling by rollingrolls was repeated 10 times. However, many holes were formed in thesheet, and the sheet could not be used as an electrode. TABLE 1 InternalThick- resist- Ca- Vol- Capacitance Volume ness ance pacitance umedensity resistance (μm) (Ω) (F) (cm³) (F/cm³) (Ω/cm³) Ex- 120 0.27 10.00.94 10.7 0.25 ample 1 Ex- 70 0.25 5.8 0.70 8.4 0.18 ample 2 Ex- 1100.30 11.9 0.89 13.4 0.27 ample 3 Ex- 90 0.26 8.1 0.79 9.0 0.22 ample 4Ex- 200 0.73 12.5 1.32 9.5 0.96 ample 5

[0047] The electrode obtained by the process of the present inventionhas low resistance and high strength and is easily formed into a thinfilm, since PTFE is formed into fibers in both length and breadthdirections to have a three-dimensional network structure. The electrodesheet is not likely to be cracked or broken even when it is a thin film,and it is excellent in shape keeping property. Further, it is possibleto obtain an elongate product by a continuous operation. Further, whencarbon black is contained as a conductive material, as high pressure isput on carbon black, the electrode sheet becomes to have low resistanceby electrical connection, even when the amount of carbon black is small.

[0048] Therefore, the electric double layer capacitor having theelectrode produced by the process of the present invention, has smallinternal resistance and large capacitance per volume.

What is claimed is:
 1. A process for producing an electrode for an electric double layer capacitor, which comprises extruding a mixture comprising a carbonaceous material, a polytetrafluoroethylene and a processing aid by screw extrusion, and rolling the obtained extruded product by rolling rolls to form it into a sheet shape.
 2. The process for producing an electrode for an electric double layer capacitor according to claim 1 , wherein the mixture contains from 1 to 50 wt % of the polytetrafluoroethylene and from 20 to 200 wt % of the processing aid, based on the carbonaceous material.
 3. The process for producing an electrode for an electric double layer capacitor according to claim 1 , wherein the extrusion temperature for the screw extrusion is from 5 to 300° C.
 4. The process for producing an electrode for an electric double layer capacitor according to claim 1 , wherein the extrusion pressure for the screw extrusion is from 2 to 300 kg/cm².
 5. The process for producing an electrode for an electric double layer capacitor according to claim 1 , wherein the compression ratio in the screw extrusion is from 1.0 to 4.0.
 6. The process for producing an electrode for an electric double layer capacitor according to claim 1 , wherein the rolling temperature is from 20 to 350° C.
 7. The process for producing an electrode for an electric double layer capacitor according to claim 1 , wherein after rolling the extruded product by rolling rolls to form it into a sheet shape, the formed product of sheet shape is monoaxially or multiaxially stretched from 1.01 to 5.0 times the original length and further rolled by rolling rolls.
 8. The process for producing an electrode for an electric double layer capacitor according to claim 1 , wherein the carbonaceous material comprises an activated carbon having a specific surface area of from 200 to 3,500 m²/g and a particle size of from 0.1 to 100 μm, and a carbon black having a specific surface area of from 200 to 1,500 m²/g and a particle size of from 0.001 to 1 μm.
 9. The process for producing an electrode for an electric double layer capacitor according to claim 1 , wherein the extruded product is rolled by rolling rolls in the same direction as the screw extrusion direction.
 10. The process for producing an electrode for an electric double layer capacitor according to claim 1 , wherein as soon as the electrode is formed into a sheet shape by rolling by rolling rolls, it is continuously bonded and integrated with a current collector made of an aluminum foil.
 11. The process for producing an electrode for an electric double layer capacitor according to claim 10 , wherein the electrode and the aluminum foil are bonded by means of an electroconductive adhesive, and the electrode and the aluminum foil bonded are rolled by rolling rolls.
 12. An electrode for an electric double layer capacitor, which is prepared by extruding a mixture comprising a carbonaceous material, a polytetrafluoroethylene and a processing aid by screw extrusion, and rolling the obtained extruded product by rolling rolls to form it into a sheet shape.
 13. The electrode for an electric double layer capacitor according to claim 12 , which has a tensile strength of at least 1.5 kg/cm² in at least one direction.
 14. The electrode for an electric double layer capacitor according to claim 12 , wherein the electrode contains from 1 to 50 wt % of the polytetrafluoroethylene, based on the carbonaceous material.
 15. The electrode for an electric double layer capacitor according to claim 12 , wherein the carbonaceous material comprises an activated carbon having a specific surface area of from 200 to 3,500 m²/g and a particle size of from 0.1 to 100 μm, and a carbon black having a specific surface area of from 200 to 1,500 m²/g and a particle size of from 0.001 to 1 μm.
 16. An electric double layer capacitor, which has an electrode prepared by extruding a mixture comprising a carbonaceous material, a polytetrafluoroethylene and a processing aid by screw extrusion, and rolling the obtained extruded product by rolling rolls to form it into a sheet shape.
 17. The electric double layer capacitor according to claim 16 , wherein the electrode has a tensile strength of at least 1.5 kg/cm² in at least one direction.
 18. The electric double layer capacitor according to claim 16 , which has an electrode assembly having the electrode bonded with a current collector made of an aluminum foil. 